Hello Mauri --
Your message brings up two points:
1. Using antennas at various heights to measure ground conditions at the
installation site will help to more accurately model the
impedances of antennas installed at that location.
2. Using antennas at various heights to measure ground conditions at the
installation site may be somewhat less helpful in modeling
radiation patterns, especially if ground conditions vary significantly within
the beam-forming zone around the antenna. For low
radiation angles or sloping terrain, the beam-forming zone can be vary large
(kilometers).
Software that I've seen doesn't contain any mechanism for entering variable
ground characteristics. And only certain software
(TA for yagis) can deal with non-flat terrain. The inability to characterize
the shape of the terrain around the site probably
introduces a much larger error in pattern determination than local variation in
ground conditions for a majority of locations.
Models have made great progress... but work remains to be done!
-- Eric K3NA
-----Original Message-----
From: owner-antennaware@contesting.com
[mailto:owner-antennaware@contesting.com]On Behalf Of i4jmy@iol.it
Sent: 2001 July 20 Fri 07:20
To: gabnjb@earthlink.net
Cc: eric@k3na.org; antennaware@contesting.com
Subject: Re: [antennaware] question on ground system for good ground
Just a series of thoughts ...
In case of horizontally polarized antennas placed at a reasonable
height, the ground costants have a neglegible impact on the radiation
pattern and efficiency, while the model should better take in account
the soil profile rather than conductivity or dielectric constant.
In case of vertically polarized antennas I've instead some doubts on
results and reliability of common amateur software over poor
conductivity media, either because of inefficient ground slope modeling
and also because of electrical discontinuities between different soil
types.
Expecially in case of vertically polarized antennas infact, what's
exactly below the antenna can be always overcome by an artificial
ground made of radials or screens (in term of losses) but what's far
away is instead what determines if and how there will be reflection,
refraction or absorbment at a stated frequency.
In the case one believes to be in an area with constant ground
parameters, than any measure or chart can be a good choice.
The noticeable advice that a soil practically good on 1.8 Mhz can be
instead rather awful a few octaves above is surely something to keep in
mind when modeling, and not only when evaluating the ground by
measuring impedance on a sampling horizontal antenna at different
heights.
73,
Mauri I4JMY
> ---------- Initial message -----------
>
> From : owner-antennaware@contesting.com
> To : <eric@k3na.org>, <antennaware@contesting.com>
> Cc :
> Date : Thu, 19 Jul 2001 22:44:02 -0400
> Subject : Re: [antennaware] question on ground system for good ground
>
>
> NEC-2 should work down to a very low height (inches). If you want to
model
> the antenna on or under the surface, you'll need NEC-4!
>
> Eric is right about frequency dependence -- my own trial was at 40M.
I was
> forced to figure out the conductivity in order to tune a 40M beam at
25 feet
> and get it to work as desired at 75 feet. The data correlates
acceptably on
> 80 and 160; I changed Beverage terminations and obtained much cleaner
> patterns.
>
> By the way, the FCC graphs show this area (30 mi. East of Atlanta) to
have a
> conductivity of 2 to 4 mS/m. Under my station, there is zero to a
few feet
> of red clay on top of granite bedrock (I have an exposed 40x100 ft.
granite
> ledge just South of the tower). Localized conductivity is less than
1 mS/m.
>
> 73, Gary
> K9AY
> ----------------------
>
> From: "Eric Scace K3NA" <eric@k3na.org>
>
> > The choice of elevations to make measurements is frequency
dependent.
> Pick a "low" and "high" height that are appropriate to the
> > frequency band being considered.
> >
> > I'm not personally familiar with the computation limitations for
NEC-2,
> but know that there is some point which is "too close to
> > ground" to give accurate results. The "low" height should be above
any
> such limit for the calculating engine.
> >
> > Lastly, don't be surprised if ground characteristics are
different for
> different bands. RF penetration can be frequency
> > dependent.
>
>
>
> --
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>
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